This invention relates to antenna structure for radiation of electromagnetic energy that is propagating along guided wave structure. More specifically, this invention relates to an antenna element for circularly polarized radiation of a predetermined portion of an electromagnetic wave that is propagating along a waveguide.
Numerous communications, tracking and telemetry systems require antenna elements for radiation of circularly polarized electromagnetic waves. In a wide range of relatively high frequency transmission (and reception) systems, the preferred arrangement is one in which the electromagnetic energy to be radiated propagates along a waveguide and the antenna element is formed as an integral part of the waveguide. In the prior art, attempts have been made to provide such structure by machining openings in the broad face of an air-filled rectangular waveguide with each opening being either circular in geometry or being a "cross-slot" that is formed by two narrow slots that intersect one another to form four equal-length orthogonally extending arms. As is disclosed in a technical article entitled "Circularly Polarized Radiators," by A. J. Simmons, IRE Transactions of Antenna and Propagation, pages 31-38, January 1956, such circular slots and cross-slot arrangements will radiate electromagnetic energy of circular or near-circular polarization when the center of the slot is positioned in the broad face of the waveguide at a point that lies between the centerline of the waveguide and one of the waveguide sidewalls.
Although prior art circular and cross-slot waveguide antenna elements can be satisfactory in some situations, various problems and drawbacks can be encountered. For example, in aerospace applications and others, size and weight constraints often exist that make it impossible or at least undesirable to utilize air-filled waveguides. In addition, in such applications, it is often necessary to configure the antenna so that the broad face of the waveguide that contains one or more antenna elements either forms or conforms to the outer surface of an aircraft, missile or other type of aerospace vehicle. Even if it is possible to configure an air-filled waveguide to the desired contour or shape, relatively complex and costly fabrication and/or forming techniques are required.
Although it is well known that the size of a rectangular waveguide for use at any particular band of frequencies can be substantially reduced by filling the interior of the waveguide with a material that exhibits a dielectric constant greater than that of air, the prior art circular and cross-slot antenna elements have not proven to be fully satisfactory in such an arrangement. In particular, circular slots in a dielectrically-filled rectangular waveguide exhibit satisfactory circular polarization, but cannot be configured for radiation of more than approximately 30% of the electromagnetic energy that propagates along the waveguide. On the other hand, when cross-slots are provided in the broad face of a dielectrically-filled rectangular waveguide, on the order of 95% of the incident electromagnetic energy can be radiated, but the radiation becomes highly elliptical. There are many applications in which the radiation characteristics of the prior art slots of circular and cross-slot geometry cannot meet system design constraints. Further, there are many applications which require that only a controlled portion of incident electromagnetic energy be radiated, with the remaining portion of the energy propagating beyond the antenna element for radiation by other antenna elements (array applications) or for utilization by other components of a microwave system.